2-acrylamido-2-methylpropane sulfonic acid is a white crystalline solid having a melting point of about 185.degree. C., a molecular weight of 207 and a Neutralization Equivalent of 207. It is commercially available from Lubrizol Corporation, Wickliffe, Ohio, and is sold by that corporation under the trademark AMPS. The acid is a monomer which is generally polymerized for a variety of uses. One such use is in medical electrodes particularly adapted to tape-like configurations for use in sensing and stimulation applications in which the electrode is applied to the skin. In such a use, one electrode component comprises 2-acrylamido-2-methylpropane sulfonic acid in a polymerized form. The polymer, in the solid solution or gel form, possesses electrically conductive properties, flexible properties and adhesive properties, particularly lending itself to skin contact and adhesion.
As presently available in a "reaction" grade from Lubrizol, the acid contains trace amounts of alkyl sulfonic acids which are believed to act as chain transfer agents, reducing ultimate molecular weight achieved during polymerization.
Lubrizol Corporation has on occasion made the acid available in a "refined" grade. In preparing the "refined" grade according to the Lubrizol procedure, the acid is subjected to a refining method consisting of: dissolving the acid in methylalcohol at elevated temperatures on the order of 175.degree.-180.degree. F., filtering the solution to remove any undissolved matter and recrystalizing the acid from the solution. Polymers from the "refined" grade exhibit higher molecular weights and improved properties.
It is a purpose of this invention to provide an improved method of refining reaction grade 2-acrylamido-2-methylpropane sulfonic acid which has not previously undergone the dissolution and recrystallization steps used in the Lubrizol refining procedure referred to above.